The increasing use of wireless telephones and personal computers has led to a corresponding demand for advanced telecommunication services that were once thought to only be meant for use in specialized applications. In the 1980""s, wireless voice communication became widely available through the cellular telephone network. Such services were at first typically considered to be the exclusive province of the businessman because of expected high subscriber costs. The same was also true for access to remotely distributed computer networks, whereby until very recently, only business people and large institutions could afford the necessary computers and wireline access equipment. As a result of the widespread availability of both technologies, the general population now increasingly wishes to not only have access to networks such as the Internet and private intranets, but also to access such networks in a wireless fashion as well. This is particularly of concern for the users of portable computers, laptop computers, hand-held personal digital assistants and the like who would prefer to access such networks without being tethered to a telephone line.
There still is no widely available satisfactory solution for providing low cost, broad geographical coverage, high speed access to the Internet, private intranets, and other networks using the existing wireless infrastructure. This situation is most likely an artifact of several unfortunate circumstances. For one, the typical manner of providing high speed data service in the business environment over the wireline network is not readily adaptable to the voice grade service available in most homes or offices. Such standard high speed data services also do not lend themselves well to efficient transmission over standard cellular wireless handsets. Furthermore, the existing cellular network was originally designed only to deliver voice services. As a result, the emphasis in present day digital wireless communication schemes lies with voice, although certain schemes such as CDMA do provide some measure of asymmetrical behavior for the accommodation of data transmission. For example, the data rate on an IS-95 forward traffic channel can be adjusted in increments from 1.2 kbps up to 9.6 kbps for so-called Rate Set 1 and in increments from 1.8 kbps up to 14.4 kbps for Rate Set 2.
Existing systems therefore typically provide a radio channel which can accommodate maximum data rates only in the range of 14.4 kilobits per second (kbps) at best in the forward direction. Such a low data rate channel does not lend itself directly to transmitting data at rates of 28.8 or even 56.6 kbps that are now commonly available using inexpensive wireline modems, not to mention even higher rates such as the 128 kbps which are available with Integrated Services Digital Network (ISDN) type equipment. Data rates at these levels are rapidly becoming the minimum acceptable rates for activities such as browsing web pages. Other types of data networks using higher speed building blocks such as Digital Subscriber Line (xDSL) service are just now coming into use in the United States. However, their costs have only been recently reduced to the point where they are attractive to the residential customer.
Although such networks were known at the time that cellular systems were originally deployed, for the most part, there is no provision for providing higher speed ISDN- or xDSL-grade data services over cellular network topologies. Unfortunately, in wireless environments, access to channels by multiple subscribers is expensive and there is competition for them. Whether the multiple access is provided by the traditional Frequency Division Multiple Access (FDMA) using analog modulation on a group of radio carriers, or by newer digital modulation schemes that permit sharing of a radio carrier using Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA), the nature of the radio spectrum is that it is a medium that is expected to be shared. This is quite dissimilar to the traditional environment for data transmission, in which the wireline medium is relatively inexpensive to obtain, and is therefore not typically intended to be shared.
Other considerations are the characteristics of the data itself. For example, consider that access to web pages in general is burst-oriented, with asymmetrical data rate transmission requirements. In particular, the user of a remote client computer first specifies the address of a web page to a browser program. The browser program then sends this web page address data, which is typically 100 bytes or less in length, over the network to a server computer. The server computer then responds with the content of the requested web page, which may include anywhere from 10 kilobytes to several megabytes of text, image, audio, or even video data. The user then may spend at least several seconds or even several minutes reading the content of the page before requesting that another page be downloaded. Therefore, the required forward channel data rates, that is, from the base station to the subscriber, are typically many times greater than the required reverse channel data rates.
In an office environment, the nature of most employees"" computer work habits is typically to check a few web pages and then to do something else for extended period of time, such as to access locally stored data or to even stop using the computer altogether. Therefore, even though such users may expect to remain connected to the Internet or private intranet continuously during an entire day, the actual overall nature of the need to support a required data transfer activity to and from a particular subscriber unit is actually quite sporadic.
Problem Statement
What is needed is an efficient scheme for supporting wireless data communication such as from portable computers to computer networks such as the Internet and private intranets using widely available infrastructure. Unfortunately, even the most modern wireless standards in widespread use such as CDMA do not provide adequate structure for supporting the most common activities, such as web page browsing. In the forward and reverse link direction, the maximum available channel bandwidth in an IS-95 type CDMA system is only 14.4 kbps. Due to IS-95 being circuit-switched, there are only a maximum of 64 circuit-switched users that can be active at one time. In practicality, this limit is difficult to attain, and 20 or 30 simultaneous users are typically used.
In addition, the existing CDMA system requires certain operations before a channel can be used. Both access and traffic channels are modulated by so-called long code pseudonoise (PN) sequences; therefore, in order for the receiver to work properly it must first be synchronized with the transmitter. The setting up and tearing down of channels therefore requires overhead to perform such synchronization. This overhead results in a noticeable delay to the user of the subscriber unit.
An attractive method of increasing data rate for a given user is the sharing of channels in both the forward and reverse link direction. This is an attractive option, especially with the ease of obtaining multiple access with CDMA; additional users can be supported by simply adding additional codes for the forward link, or code phases in the reverse link for an IS-95 system. Ideally, this subchannel overhead would be minimized so that when additional subchannels need to be allocated to a connection, they are available as rapidly as possible.
To maintain synchronization, it is therefore advantageous to provide the subchannels in such a way that the lowest possible speed connection is provided on a reverse link while at the same time maintaining efficient and fast ramp-up of additional code phase channels on demand. This in turn would maximize the number of available connections while minimizing the impact on the overall system capacity.
The present invention is a service option overlay for an IS-95-like CDMA wireless communication system which accomplishes the above requirements. In particular, a number of subchannels for a forward link are defined within a single CDMA radio channel bandwidth, such as by assigning different orthogonal codes to each sub-channel. Multiple subchannels are defined on the reverse link by assigning different code phases of a given long pseudonoise (PN) code to each subchannel. The instantaneous bandwidth needs of each on-line subscriber unit are then met by dynamically allocating none, one, or multiple subchannels on an as needed basis for each network layer connection.
More particularly, the present invention efficiently provides a relatively large number of virtual physical connections between the subscriber units and the base stations on the reverse link for extended idle periods such as when computers connected to the subscriber units are powered on, but not presently actively sending or receiving data. These maintenance subchannels permit the base station and the subscriber units to remain in phase and time synchronism in an idle mode and also request additional channels. This in turn allows fast acquisition of additional subchannels as needed by allocating new code phase subchannels. Preferably, the code phases of the new channels are assigned according to a predetermined code phase relationship with respect to the code phase of the corresponding maintenance subchannel.
In an idle mode, the subscriber unit sends a synchronization or xe2x80x9cheartbeatxe2x80x9d message on the maintenance subchannel at a data rate which need only be fast enough to allow the subscriber unit to maintain synchronization with the base station. The duration of the heartbeat signal is determined by considering the capture or locking range of the code phase locking circuits in the receiver at the base station.
For example, the receiver typically has a PN code correlator running at the code chip rate. One example of such a code correlator uses a delay lock loop consisting of an early-late detector. A loop filter controls the bandwidth of the loop which in turn determines how long the code correlator must be allowed to operate before it can guarantee phase lock. This loop time constant determines the amount of xe2x80x9cjitterxe2x80x9d that can be tolerated; phase lock is typically considered to be maintainable when this is equal to a fraction of a chip time, such as about xe2x85x9 of a chip time.
The heartbeat messages are preferably sent in time slots formed on the subchannels defined by the code phases. The use of time slotting allows a minimum number of dedicated base station receivers to maintain the idle reverse links. In particular, the reverse maintenance channel links are provided using multiple phases of the same long code as well as by assigning a time slot on such code to each subscriber unit. This reduces the overhead of maintaining a large number of connections at the base station.
Because of the time slotted nature of the reverse maintenance channel, the base station receiver can also be time shared among these various reverse links. To permit this, during each time slot allocated to a particular subscriber unit, the base station receiver first loads information concerning the last known state of its phase lock such as the last known state of early-late correlators. It then trains the early-late correlators for the required time to ensure that phase lock is still valid, and stores the state of the correlators at the end of the time slot.
When additional subchannels are required to meet bandwidth demand, the additional code phases are assigned in a predetermined phase relationship with respect to the locked code in order to minimize overhead transmissions which would otherwise be needed from the base station traffic channel processor. As a result, many thousands of idle subscriber units may be supported on a single CDMA reverse link radio channel while at the same time minimizing start up delay when channels must be allocated.